Oscillation generation and control



Ma s, 1942.

c. w. HANSELL OSCILLATION- GENERATION AND CONTROL Filed Nov. 16, 1938 4 Sheets-Sheet 1 All ll in uwu zq umkeq INVENTQR C. W. HANS v ATTORNEY May 5, 1942. c. w. HANSELL OSCILLATIONGENERATION AND CONTROL I Filed Nov. 16, 1938 4 Sheets-Sheet 2 INVENTOR C. W. HANSELL ATTORNEY M y 5, 1942- I c. w. HANSELL I ,2

OSCILLATION GENERATION AND CONTROL Filed Nov. 16, 1938 4 Sheets-Sheet 3 1 Fig.3

T R- 329 g 332 3H =7 SOURCE OF MODULATION PO WER 5 RAID/0 FREQUENCY W RECTIFIER 322 38 J RECTIFIER 4 0R BATTERY v2 PUSH-PULL v LONG L/NE CONTROLLED L 1 T OSCILLATOR AUDIO MODULATION INPUT 40/ 405 405 f I 'v L) m 03 \l\l INVENTOR c. w. HANSELL ATTORNEY May 5, 1942 c. w. HAN SELL OSCILDATION emiE ATIon AND" CONTROL 4 Sheets-Sheet 4 Fil ed NOV. l6,, 1938.

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Patented May 5, 1942 UNITED f srA'rEs PATENT OFFICE CSCILLATION GENERATION AND CONTROL Clarence W. Hansell, Port Jefferson, N. E, assignor to Radio Corporation of America, a corporation of Delaware Application November 16, 1938, Serial No. 240,629

2 Claims.

My present invention relates to the production and control of high powered short waves of constant frequency and is a continuation in part of my United States application Serial #692,092 filed October 4, 1933, now Patent #2905380, dated October 19, 1937, and of my United States application Serial #141,467, filed May 8, 1937, now Patent #2,l60,655, dated May 30, 1930.

My present application discloses a new and improved method of and means for modulating high frequency oscillations. and is particularly adapted for use with generators as described in said aforesaid applications.

In describing my invention in detail reference will be made to the attached drawings, wherein:

Figure 1 illustrates schematically the electrical circuits of a. high frequency transmitting system in which automatic frequency comparison and control with a standard source may be obtained with means for modulating the transmitted high frequency wave;

Figures 2 and'3 are modifications of my improved long" line frequency controlled transmitter especially adapted for shorter waves.

Figures 4 and 5are further modified formsof long line controlled transmitters for short waves incorporating therein modulating systems especially adapted therefor; and

Figure 4a is an end view of the line of Figure 4. In Figure 1, the-master oscillator comprises a pair of electron. discharge devices ll, 40 preferably of the water cooled variety, each having a plate or anode 42, a grid or control electrode 44, and a cathode or filament l6. Across the anodes of the vacuum tubes 38, 40, there is connected a parallel tunable circuit 48 consisting of an inductance coil 50 and a variable condenser 5|. The circuit ll acts as a rough frequency controlling circuit for controlling the frequency plained more fully in my United States Patent #1,945,546, dated February .6, 1934. Briefly, the action of the resonant llnefor frequency control 'High voltage plate supply is obtained from at its sharply resonant frequency. In another lightit may be said that waves traveling down the line are reflected back so that standing, waves on the line pull or lock the oscillations generated into step with the standings/ ives, and since the termining factor for frequency control. From of quarter wavelengths long will not give the zero phase difference, the line tends to react upon the system so as to establish the zero phase difference.

The filaments are supplied with heating potential from a source connected to lines I02 supplying filament transformers 58, it. In some cases I prefer to supply the two filament transformers 58, 60 from diflerent'phases of a two phase power supply in order to balance out muchof the disturbing effects of alternating heating current. leads 62, 64, connected to a rectifier 6B.

The neutralizing condensers as, 10 cross-connecting theanodes and control electrodes of tubes 38, 40, are adjusted soas to cause sufllcient feedback for oscillation generation. Another'featui'ement of these condensers so that the phase of the voltage on the grids is in advance, or' in other words, so that the grid voltage leads the plate voltage. This adjustment is obtained by making,

zation, I have found that there is an improve-- ment in efllciency and this may be explained by.

considering that the time of travel of electrons in the tubes tends to make the plate current and may be explained by considering the line to be' f an exceedingly sharp resonant circuit which 4 causes the oscillator to oscillate .very constantly F uplisse due to time of electron travel is a minor anced as a consequence of which more nearly optimum adjustments are obtained. This shift losses and time of electron travel. In cases where 1 the plate circuit losses are predominant, "as may I control voltages for the'grids 4| of the tubes 30,, 40, when the. line has. the desired amount of cir- 1 factor in neutralized, amplifiers: because of the I fact-thatthe relative phase of the grid and anode .voltage isnot important. However, I have found that it becomes quite an appreciable factor in oscillation generators particularly of the water substantially one-quarter wavelength, but,'.for

ferred to use as a stabilizing resonant circuit is I pushpnll use as illustrated in Figures 1 andz, it

cooled type and particularly when used-at the shorter wavelengths.

' It should also be noted that, in the line con-' Q trolled oscillator, the losses in the line constitute the greater part of the total load. If the oscillain a lagging direction and this added to the phase shift due to. time of electron travel might come I a to a considerable total. On the other hand, if 3 oscillation is obtained by over neutralization, the

310 tor were adjusted for oscillation by virtue of tinder neutralization the effective resistance in the grid 1 circuit would give the grid voltagea phase shift 1 is desirable and convenient'to use a line whose total length is one-half of the wavelength of the desired operating frequency. The half wavelength is folded up upon itself as illustrated to form a U, which from one extreme end to 'the other is approximately one-quarter wavelen th of the operating frequency. By making the line longer and in this 'particularcase' any number of half wavelengths long, its energy storage capacity is increased giving rise to, increased stability as well as reducing temperature risedue to its own losses. I The amount of. energy which i a may be stored in a line is roughly proportional line losses cause a shift in grid voltage phase in a 3 leading direction so that the final resultant phase shift is the difference between that due to line to its length, and the: temperature increase for I a given storage of energy is roughly inversely proportionalto its length. For most practical purposes I prefer to make the long line system one-half wavelength in length. I I For modulating the system shown in Figure 1.-

, be the case where'the oscillator feeds energy di- 1 g j rectlyto an antenna, it maybe preferable in some 7 cases to obtain oscillation'by under neutralization in order to obtain optimum phase relations be- 3 tween anode and grid radio frequency "voltages.

In any case I choose between over and under neutralization to obtain optimum efficiency and the choice is principally governed by the distribution of loading in the circuits.

Grid bias and modulating potentials may be supplied through conductor I2 connected to a y metallic slip ring I4 at the electrical center of thelong line 2, I, 4 which usually will befound i. at the'midpoint of the C-shaped portion! of the the plate voltage may be varied by introducin modulating potentials into the plate voltage lead 64 by means of a transformer or the grid voltage may be modulated by similarly introducing modu-. lating voltages in the grid biasing lead I2. A convenient scheme for modulating the apparatus for code signalling as illustrated is described more 17, 1936-. As illustrated in Figure 1 the oscillator gridnegative bias potential is obtained by passing the rectified grid current of the tubes through line 2, I, 4. The grids may be connected to simi- K laradjustable slip rings I6, I0 and all three rings '1 14,16, 18 may be provided with set screws which X may, if'desired, be soldered or brazed into fixed position after final adjustments have been made. i The point 16,18 should be chosen a sumcient distance away from the electrical center I4 upon the long line tube systemso as to provide the proper 1 culating radio frequency energy.

' tential to the grids 94 of the keying tubes 96,.

Preferably I make the'long, line' system including the irmer hollow piping 2, I, 4 and the outerv 1.

1 hollow pipes or cylinders 8, 8 of solidcopper. If

desired, 'ofcourse, the inner pipe may be madeof other materials with a copper tubing or a sumthe commongrid resistor 88. The transmitter is keyed by imposing a higher negative potential on fully in my Patent No. 2,060,988, dated November the oscillatorgrids from the keying circuit which consists of a key or relay 00 applying and removing negative potential from source 92 to the grids "of the'parallel connected keying tubes 96. The I two rectifier tubes 98, I00 by rectification action rectify voltages from the source connected to lines I02 for use as negative cut off bias for the two oscillator tubes 40,38. Reactor I04 and condenser I06 act to. smooth the rectified keying voltages.

When key 00 is down and applies negative 50- tubes 08 are biased to cut-oil. Consequently, the I bias rectifier l0, I00-is rendered'inoperative so that no negative potential is applied to the grids M of the radio frequency oscillator tubes, 40.

ciently heavy copper plating at the surface thereof since at the high frequencies involved most of the current will tend to flow in the outermost surface of the tubing. To further reduce losses the inner pipe or line 2, I, I may be made of presses a high negative bias upon the grids 44 of may very well have less energy losses than tar nished silver orcopper surfaces. The cost of .gOld' plating is not'prohibitive at extremely high frequencieswhere the length of line is small.

As illustrated, the legs ofthe frequency controlling line are transversely spacedand arranged parallel to each other. The outer pipes 6, 8 prevent undesired radiation from the frequency controlling lines and protect it from disturbance due 1 to other nearby equipment. I

The minimum length of line which it is pre When the key '00 is 'upthe grids 94 of tubes 96 assume filament potential as a consequence of I which the tubes 06 become conductive allowin resistor 88 asa result of which conductor I2 imthe radio .frequency oscillation generatorsuch that oscillation ceases. In passing itmay be noted that the resistance 88 is commonto both the oscillator and the keying circuit. Condenser I 00 is the'radio frequency ,by-passing condenser and the resistor H0 is provided to assist resistor 88 in establishing a grid bias for the grids N v under oscillating conditions in tubes 38, 40. The

resistor II 0 is not essential to the operation of the system but is preferred-in some cases, since it assists; in preventing spurious oscillations.

-As the long line frequency controlled oscillators such as shown in Figure 1 are often placed out in the field with inexperienced help or placed or another there may be a slight drift in frecurrent flow from rectifiers 98, I00 to flow through I in inaccessible. places, and since for some reason quency, I have provided means as shown in Figure 1-forautomatically correcting the frequency I 2,282,395 of the transmitter system soas to correspond with and closely follow some master controlling I frequency such as may be generated by a carefully controlled piece-electric crystal controlled oscillator. As shown .in Figure 1, the frequency controlling means for the long line system consists of a rectangular metallic plate 2 raised or lowered by reversible electric motor I it through and ground is increased or decreased thereby decreasing or increasing the frequency of 'operation. The motor HI isoperated by means of an electromechanical relay system HI operating a switch through leads I20 to the motor Ill.

In the long line very short wave system of Figure 2, the pushpull connected vacuum tubes 38, 40 are provided with fluid cooling or water cooling Jackets 2H. 2. The plate or anode circuit of the pushpull connected tubes 38, 40 includes the rectangular or circular flat capacity plates-2w, .218 directly metallically connected to the water cooling jackets 212, 2. Rotation of knob 220 rotates the screw and gear mechanism 222 and hence the rotation of the internally threaded member 224 in turn causing the variable grounded plates 22! to approach or recede from the condenser plates 2", 2|! simultaneously. As illustrated, condenser, 22. may be grounded for purposes of symmetry through theear mechanism and conductor 220. The inductance of the plate circuit is formed by bi-fllar- 1y wound metallic coils 222, 224 to the electricalcenter 23B of which, by means of inlet pipe 222 and outlet pipe 2, cooling fluid or water is introduced or removed from the anode Jackets 2, H2. For very high frequencies the inductance of the plate circuit may be formed of straight I is best to pass the cooling fluid through the Jack-' ets in an upward direction in order to assure that practically all air is forced out of the Jacket leaving only the fluid in contact with the anode of the tube. The coils 224, 232 which act as the unitary structure for high frequency electrical currents but has a dual path for cooling fluid, are'shown arranged so that their axes are Ill applying reversing potentials output circuits of the tubes may be varied and hence the frequency may be varied by a relatively large amount, connection of switch 2 to connectors 2, 2 acting to short circuitaway a part of the inductance of the plate circuit of the pushpull oscillation generator. Tuning variation may also be accomplished by manipulation of knob 22. hence varying the plate tuning condenser 2|, 228, 2". Output energy is taken by means of coils .250; 282 coupled respectively and inductively to the water cooled coils 234,

The coils 250, 252 may be connected tosether and to ground as at 254 and have their other ungrounded terminals connected through lay-gassing condensers 258 to a radiating antenna 2. supported within and coaxially with ,the water cooled coils 224, 232, or if desired may be wound about and insulatingly supported from the water cooled coils. If the inductance 232, 234- is formed of straight tubing. as would be the case at very.

high frequencies then the coupling 250, 252 may also be made of straight conductors placed parallel to and near the conductors 222, 224.

The grids 2", 252 of the tubes. 40 are vari- .ably connected through connectors 264, 288 to parallel to the longitudinal cylindrical axes of the cooling Jackets 2", 212. In practice I make the axes of the cooling jackets and of the straight or coiled plate circuit inductance vertical and so shaped that all the cooling fluid can drain out when the supply. of fluid from a pump is' interrupted. This prevents damage to the system from freezing of the fluid when water is used ventionof freezing I may use various well known antifree zing mixtures for the cooling liquid.

For a change in frequency, there are provided theconductors 2, 2 which are variably'tapped to the coils23l,j232, and by connectingthe con- 2 ,to the contacts 2, 248. Where. operation is desired at frequencies which are not harmoniductors 2, 2" together by means of-a -suitable switch 242 the ,eifective inductance; of the:

points 288, 210,, on either side of the electrical center 212 -of the resonant line LLF'C for frequency control. Since the system of Figure 2 contemplates the use of very' short waves, the

long line for frequency control is made in the form of a continuous flattened ellipse with two trough, bent. bottom or 0 portions 214, 216, themidpoints of each trough or elbow portion 2.14.

21 being at a voltage nodal point. Intermedipoint on the inner tubular conductor forming part'of the long line frequency control system. About the straight portions 212. 280 of the inner tubular conductor of the long line system there are suitably mounted concentric linear conduct ing tubes or pipes 222, 284 suitably cross-connected and grounded along their length so as to maintain them at zero radio frequency potential. If desired the outer conductors '222, 2" may be made continuous and completely concentric around the inner conductors 214,216, 212, 220,

but in that case openings for the various connec-- tions to the; inner transmission line 214, 220. 216.

cordance with that specified for the long line system of Figure 1 and hence for the sake of brevity will not be repeated here. The overall length of the long line system shown in Figure 2, that is to say, the mean length of the inner conductor 214, 218, 21 2; 2. should'preferably be a whole number of half wavelengths long,

including unity, at the desired operating frequency. For adjustment to difl'erent frequencies, it should be noted'that the same line would be effective for approximately harmonically related frequencies. Hence, by suitably tapping conductors 2. 246 to the water cooledcoils, quick change to a harmonically related frequency and stabilization on the same long line may be obtained' by the connection of short circuiting strap cally related, I may also provide'means for quickly. changing, the effective length of thelinev by.

The coils 250, 252 may be insulatingly.

indicated as 32 in Figure 1.

means of switching in orout sections of line, shunt or series inductance, capacity, etc.

If desired, the resonant line system may be placed within a suitable container 290 of heat insulating material and-the long line system temperature controlled. Moreover, a wall 292' may be placed across one end of the line and temperature controlling fluid fedon one side of the wall 292 through tube 294 and removed from theothe'r side ofthe wall, by tube 293, the circulation of fluid through theinner conductor being as indicated by the arrows. Suitablepump ing mechanism 293 may be provided for causing a continuous circulation of the temperature controlling fluid in the long line which may be water 'or oil'maintained at a suitable temperature or which may be. air or any gas. In case the circulating liquid or the gas is held-at a temperature impressed by conductor 323, will cause modula- 7' tion of the output of the frequency controlled system. A pair of diode rectifiers1322, 324 are coupled by'means' of theirplate coil 323 sym- Y metrically to the'plate circuit of the pushpull connected tubes 38, 40. By coupling the rectifier coil 326 sufiiciently close to the plate circuit,

enough of the radio frequency energy developed by the oscillator will be rectified and cause sumbelow that of the room or spacearound the line then'some hydroscopic material should be placed within the "heat insulating material 293 to prevent the formation of water drops upon the long line system. Such condensation, it should be noted, would tend to change the operating frequency, due to the fact that it would change the velocity of the electrical waves along the line.

As a furtheraid to short wave operation, the legs of the filaments of the tubes 33, 43 may be currents from an amplifier 332. The fluctuations A short circuited together for high frequency vcur- I rents bymeans of by-passing condensers 333. so

act in parallelfor high frequency, as do also filathat the filament heating conductors 332, 304

ment heating conductors 396, 303. At the elec trical centers 3", 30,01 the loops formed by the heatingconductors332, 334,333,333, another by-passing condenser 2 may beconnected and filament heating energy applied adjacent thereto, as for example, by feeding low frequency heating currents thereto by means of a transformer 3l4. The-conductors 332, 304,333, 303 will act, by judicious choice of their length, as a tuned filament circuit for the hizh frequency waves and theirimpedance should be so adJusted as to obtain optimum operating conditions, that;

is tosay, so that the filaments fluctuate at high frequency potential at a .desired amount and with the desired phase relation.

For modulation any of the schemes described noted in addition that the source 92 of Figure 1 may be replaced by an amplified alternating current sourcesuch'as a keyed tone source, or, by

amplified voice currents. Also source 92 may 'cient drop in the resistor Rand across radio frequency by-pass condenser C so as to limit the oscillator output. I

In addition to this rectifier system for obtaining gridbias, there is serially connected a bat- 'tery 323 which may be replaced by a potentiometer resistor supplied with voltage from a 60 cycle rectifier system. This second source of negative Y potential 328 serves to adjust the circuits for best operating characteristics. 1 1

In operation, thetwo sources of grid bias,

namely, source 323 and the bias developed by rectifiers 322, 324 should be adjusted to limit the oscillations developed by the pushpull amplifiers 38, 40 to about half. of the maximum amplitude.

In series with the bias sources there is connected the secondary of a low frequency transformer33li supplied with amplified tone currents or voice in grid bias due to the alternating voltages developed inthe secondary of transformer 330 will then vary the output from the oscillator in ac cordance with modulation.

The advantage of this modulation system re-' sides in' the relatively small size ofthe modu1a-- tion equipment and also in its relatively small cost. 1

The plate lead 242 is shown grounded for radio frequency currents by means of a by-passing condenser -334.- It may be found desirable in sup-' in, connection wlthl 'igurei may be used, it being modification is shown in Figure 3.

odes will then be supplied with direct grid biasdenser 334.

If desired, I- may modify the modulating system D of Figure 2 by using triode tubes322' and 324 in place of the diodes shown at 322 and 324. This The triing potential either. from a separate source or by connected between the cathodes and ground or consist of a rectifiersupplied with amplified and keyed or modulated relatively low frequency current. For example, "in one application of my invention, telegraph signals are-sent, from. an

automatic senderin a central telegraph oiilce,

j over a wire line, as one channel of a number of channels carried-by the same line, to a radio transmitting station. f At the radio station, filters separate the signals of the several channels by frequency discrimination. The energy of one or more channels is then amplifiedand rectified to supply the input .to a transmitter at the' polnt As the service requires, the automatic telegraph ,sender in the central ofiice may be replaced by automatic printer mechanism or by a facsimile transmitter.

A,desirable' scheme for. modulating a .line conof source323 when -323.is used, biases, tubes 33.

trolled system is-illustra't'ed inFlgure 2'. Grid bias is fed through conductor :20 to the electrical center 2.12 of the long line'and thence through conductors 234. 233 to the grlds233', 232 of vacuum tubes)", 43-." Variation in the grid biases means of parallel resistance and capacity 329 source 323. The grid biasing potential to the triodes is then so adjusted that the drop across resistance R, or this drop plus the potential 43 to such an. extent that approximately hall maximum output current to the antenna is ob: taincd. The output 'of amplifier 332 is then applied to the grids of the triodes' 322', 324' which i Under these conditions V the oscillator output will be modulated in ac-- replace diodes 322, 324.

cordance with the output from amplifier 332.

in the long line frequency controlled system of Figures 4 and 41;, the reversible motor 1 [4 drives a triangular grounded metallic plate 333 back i and forth, so thatthecapacity between this plate;

and the end plates 333,. 343 connected to the free electrical ends :42, m or theinner transmission line 343,, 343, 333 is varied. "The outer tubes or pipes 352, 354 are connected together and grounded as indicated and are used to shield theiinner legs 343, 353'of the u shap'ed frequency controlling tubular metallic transmission line.,

As before the grids of the tubes 33, 43 are connected by means of conductors 264, 266' on opposite sides of the electrical center 212 of the inner concentric conductor. In addition to the neutralizing or feedback control condensers 366, 363, there are provided feedback condensers consisting of inner and outer cylinders 363, 362 and 364 and 366, tube 363 being insulatingly supported from tube 362 and tube 364 being insulat ingly supported from tube 366. By moving the tubes 363, 364 in and out of tubes or pipe sections 362, 363, an adjustment in addition to the feedback adjustment given by condenser 366, 363 may be had. The plate tuning condenser is diagrammatically shown at 363 connected between the water cool- I ing jackets H2, 214 of vacuum tubes 43, 33. The

plate inductance coil in this instance consists of a single doubly wound metallic'tube 313 provided with inlet and outlet water cooling ducts 312. Obviously the plate inductance may be made of straight tubes 'for very high frequency operation. Output energy may be taken from blocking condensers 314 and 'fed directly into an antenna or to any subsequent amplifier as found desirable. Plate potential is fed from conductor 242 through two choke coils 314, 316 to points 313, 333 on opposite sides of the electrical center of the coil 313 which center is preferably made between the inlet and outlet pipes 312. By closing switch 332, a section of the water cooled inductor 313 may be short circuited out of circuit causing, if desired, operation at some harmonic frequency. This construction is often desirable where mechanical considerations restrict the location of tapping. points 313, 333. for, by adjust-' ing the size of coils 314, 313 and the inductance of connections through the switch 332,it is often possible to adjust the circuits to either of two desired operating frequencies much more readily than can be done by varying taps 313, 333 alone. The line for frequency control is well adapted as explained in my above mentioned United States Patent No. 1,945,546, for control at either the fundamental or any harmonic frequency. It should also be noted that, if desired, I may provide more than one frequency controlling line and connect them to the grids of the tubes alternately or in various combinations to obtain various frequencies and conditions of operation. The changing of connections may be done by manual changing of connections or by means of selective switches as desired. In some cases one operating handle or mechanism will simultaneously operate switch 332 and change frequency controlling lines.

For modulation purposes voice currents from an audio frequency amplifier 334 are impressed upon the transformer 333 corresponding to the transformer 333 of Figure 2. In place of the battery 323 of Figure 2, however, a rectifier 333 has been illustrated in Figure 4.

a comprisestubes 33 and 43, having their anodes connected in a high frequency circuit 433 and their grids connected as in the prior modifications to a line 43!. The rectifiers 322 and 324 r are coupled at 326 to the output circuit 433.

The cathodes are in series with a source of potential 433 connected at one terminal to the cathodes of tubes 33 and 43. The grid bias circuit is completed by connecting a symmetrical point on circuit 326 to a symmetrical point on circuit 43| by line 436. This connection includes the secondary of a modulation transformer 436.

Here, as in the prior modifications. some of the radio frequency energy from the oscillation circuit 433 is rectified in rectifiers 322 and 324 to obtain a negative grid bias voltage for the grids of the oscillator tubes 33 and 43. It the rectifier is coupled at its output close enough to the circuit 433 the bias potential developed in the rectifier -will limit the amplitude of oscillations. If desired, the source of rectified current 433 may be connected in series with the rectifier comprising tubes 324 and 326 and excited by a 60 cycle power system. This second source ofnegativepotential servesto adjust the circuits and potentials for best operation. 1

In operation the two sources of grid bias potentials, that is. the source produced in the rectifier comprising 'tubes 322 and 324,- and the source derived from the 60 cycle power system, are adjusted to limit the oscillations to about half-oi maximum amplitude. In series with the bias sources is the secondary of the modulation transformer 433. The fluctuations in grid bias due to the modulations then varies the output of the oscillator in accordance with the said modulations.

The dimensioning of the line system of Figure I 4 should be in accordance with the statements expounded for Figures 1 and 2 and the long linesystemmaybesupportedifdesiredas shown in Figures 1a and lb or may be suspended quencyosciilatoro! theioncnmcontmueetvm In favor of the system as described in connection with Figure 5, is that the equipment rea high frequency generator comprising, a pair. of electron discharge devices each having a control grid, an anode and a cathode, a tuned output circuit connected to said anodes and cathodes, a frequency determining line connected to said control grids and cathodes. said devices, tuned circuit and line cooperating to produce oscillations of a frequency, determined roughly by said tuned circuit and more accurately by said line, a rectifier coupled to said tuned circuit, means includin an impedance connecting said rectifierin a direct-current circuit between the control grids and cathodes of said devices for rectifying produced oscillations and supplying/a direct-current biasing potential to said control grids to limit the amplitude of the oscillations generated, and means for superimposing on said biasing potential a modulating voltage to thereby modulate the amplitude of the oscillations generated.

2. A system-as recited in claim 1 wherein said rectifier is of the full wave type and wherein said p dance is of such a. value that in the absence of modulating voltages the amplitude of the oscillations produced is limited to about one-half of the maximum amplitude of the oscillations the oscillator is capable of producing.

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